The light and dark reactions of photosynthesis in cyanobacteria involve massive macromolecular assemblies, the Phycobilisome and the carboxysome, respectively.  Phycobilisomes harvest light energy and funnel it to the photosystems for the generation of ATP and reducing power that is used in the dark reactions.  Carboxysomes are bacterial organelles that contain Rubisco, the CO2 fixing enzyme of the Calvin Cycle.  Encapsulation of Rubisco in the carboxysome shell protects it from oxygen, its competitive inhibitor generated in the light reactions.  Carboxysomes exemplify the basic structure and function of Bacterial Microcompartments (BMCs).  BMCs are widespread among Bacteria; they are multienzyme-containing organelles bounded by a selectively permeable protein shell. In general, BMCs sequester segments of metabolic pathways, protect oxygen-sensitive enzymes, and sequester toxic and/or volatile intermediates.  By combining structural studies with bioinformatics and molecular and synthetic biology approaches, we are learning the structural basis of function and regulation of phycobilisomes and of BMCs that extends from the role of individual proteins to mesoscale organization.   This understanding has applications in the engineering of natural and artificial photosynthesis and in the development of programmable metabolic modules and biomaterials for bioengineering and nanomedicine.